Make Bee config break by default

[j2ein]

Is your feature request related to a problem? Please describe.
Currently, the bee config is written in a way that the default config without a valid registration token is valid and can be used.

Describe the solution you'd like
It would be better to write it in a way that it breaks, or give a meaningful error message when the registration token is the default.

Describe alternatives you've considered
None.

[felixauringer]

In #25, I added a new log message to make it easier to find the problem. However, I would argue that the real problem is the Internal Server Error that the API returns if the token is not valid. It should return a more meaningful error so that we can be sure (and log) that the problem is indeed the token.